Graphitic steel



graphitic steels.

Patented May 19, 1942 GRAPHITIC STEEL Frederick R. Bonte, CantomOhio,assignor to The- Timken Roller Bearing Company, Canton, Ohio, acorporation of Ohio No Drawing. Application July 26, 1940,

Serial No. 347,767

, '1 Claims. This invention relates to improvements in Graphitic steelsas made heretofore in accordance with the invention of Patent No.2,087,764, granted July 20, 1937, on an application filed by me, haveproved themselves admirably adapted, unusually so in some instances, tovarious uses, and consequently not only have they been made and sold inlarge amounts, but also their uses have been progressively increasing.For the most part the graphitic steels used commercially have been oftwo types. One of these, a water hardening type, generally containsabout 1.5 per cent of carbon and about 1 per cent of silicon; the other,which is hardened by oil quenching, is of similar composition butcontains a small amount of molybdenum as taught in accordance with thedisclosure of the aforesaid patent.

Despite the decidedly advantageous properties of such graphitic steels,extended experience in their use has shown that neither of thecommercially used types, nor any other of the specific compositionsdescribed in my aforesaid patent, is adapted to uses involving operationat high pressures and under severe abrasive conditions such, for exampleas encountered in cold drawing dies, deep drawing dies, work blades ofcenterless grinders and the like. The previously used graphitic steelsas ordinarily produced for other purposes are not adapted to these usesbecause they are deficient in wear resistance, and it has not beenpossible up to the time of this invention to adapt them for suchpurposes by any heat treatment.

It is among the objects of this invention to provide graphitic steelarticles which are especially adapted for uses involving high pressureand severe abrasion, which may be made easily and readily, which combinethe advantageous lubricarting properties of ordinary graphitic steels asknown heretofore with the ability to be hardened to provide higher wearresistance than heretofore possessed by graphitic steels.

In accordance with this invention graphitic steel articles are made fromsteels containing from about 1 to about 2 per cent of carbon, about 0.3to 2.0 per cent of molybdenum, about 0.4 to 0.8 per cent of silicon, andabout 2.0 to 6.0 per cent of tungsten, Within such ranges it ispreferred for many purposes to form the articles from steels containingabout 1.4 to 1.6 per cent of carbon, about 0.4 to 0.6 per cent ofmolybdenum, about 0.6 to 0.65 per cent of silicon, and about 2.6 to 3.0per cent of tungsten. The remainder of the steels is iron together withmi. 14s- -2) impurities in the amounts customarily encountered in suchsteels, although there may be present other alloying elements providedthey do not detrimentally afiect'the properties derivable through thepractice of the invention, for which reason the remainder of the alloymay be said to be effectively iron inasmuch as such additional alloyingelements do not alter the essential character of the products.

For the purposes of this invention the content of silicon in the steelsis somewhat less than that of the graphitic steels made and usedheretofore. In the articlesprovided by the invention it is desirable tohave a certain amount of graphitic carbon present in the structure, toconfer its lubricating properties, while having the remainder of thecarbon present as carbides to provide the desired abrasion resistance,Hence the silicon is reduced in view, of the graphitizing effect ofmolybdenum and tungsten so as to avoid graphitization to an undesirablygreat extent.

I have found'that both molybdenum and tungsten are essential to thepractice of the invention although it is sometimes considered in steelpractice that these two elements are more or less interchangeable, dueregard being had to proportions. Such equivalency or interchangeabilityis not present in the present invention. Molybdenum contributes to theability to harden the products adequately at the surface, i. e., toachieve high hardness and shallow hardening. For this purpose it cannotbe replaced by tungsten, thus, for the purposes of the inventiontungsten cannot be eliminated by the use vof larger amounts ofmolybdenum because when that substitution is made the desired wearresistance is not developed. On the other hand, molybdenum cannot bereplaced by adjusting the amount of tungsten because, as experience hasshown, when. molybdenum is eliminated from the composition the desiredhardening ability is not attained.

The steels are made in accordance with practice standard in the art forthe production of graphitic steels. Preferably they are made in anelectric furnace following standard killed steel practice, themolybdenum and tungsten being suitably introduced by furnace additionsof ferro-molybdenum and ferro-tungsten.

It has been found that the best results are obtained by stripping theingots from the molds only after the ingots have reached black heat, andthen to permit the ingots to cool to atmospheric temperature beforeplacing them in the soaking pits. ingots are stripped at red heat, sayat 1300 to Experience has shown that if the 1400 F., and put directlyinto the soaking pits, graphitization may occur which will interferewith proper working of the ingot. Or,'if the ingots are stripped blackand placed in the soaking pits cracking may occur during the working.For this reason the ingots are stripped black, allowed to cool toatmospheric temperature, and then reheated, suitably by soaking at about2000" F.

After reheating the ingots are worked hot, as'by rolling or forging,suitably in the manner in which other graphitic steels are worked, carebeing taken that during the hot working the material does not cool belowabout 1600 F.

The shaped articles are then graphitized followed by hardening. To theseends they are first normalized by heating above the critical range,suitably at about 1700" F., to cause decomposition and diffusion ofcarbides, especially carbides segregated in the grain boundaries, afterwhich they are cooled in accordance with ordinary normalizing practice,Thereafter the articles are reheated into or above the critical range,say at 1400 F., followed by cooling at a slow rate, advantageously about40 F., per hour, to a temperature well below the critical range,suitably 900 F., after which they are removed from the furnace andallowed to air cool. In this manner, the carbides are partiallydecomposed with production of graphitic carbon and with spheroidizationof residual carbides, the combination of which gives the propertiesdesired together with uniform response to heat treatment.

A treatment as just described of steels made from the preferredcomposition stated above will contain from 0.3 to 0.5 per cent ofgraphitic carbon, with the remainder of the carbon in combined form.Such an amount of graphitic carbon provides surface lubricatingqualities desirable in articles provided by the invention, while thespheroidized carbides provide, upon suitable heat treatment, high wearresistance. The combination of graphitic carbon and spheroidizedcarbides also causes the steels in this condition to be easilymachinable. All of these properties are further enhanced by the factthat in steels made and treated in accordance with the invention, thegraphitic carbon is much more finely dispersed than is the case with thepreviously used graphitic steels, and it is distributed highly uniformlythroughout the entire structure.

After being graphitized the articles are machined or otherwise finishedto shape and size, after which they are subjected to a hardeningtreatment in which they are heated above the critical range andquenched. Solid sections may, for example, be quenched into water orbrine from about 1450 to 1500 F., the exact temperature and quenchantvarying according to the section of the article and the particularsurface hardness desired. Less uniform sections may be quenched in oilfrom about 1550 F. This heat treatment does not cause the graphiticcarbon to return to the combined state, which is obviously advantageous.By heat treatment in this manner surface hardnesses as high as 69 to 70-Rockwell C may be obtained by water or brine quenching, while somewhatlower hardnesses, about 66 or 67 Rockwell C, result from quenching inoil.

The presence of extremely finely dispersed graphitic carbon and ofcarbides in the structure of articles produced in accordance with thisinvention contribute to provide, to repeat, properties which render thearticles particularly adapted to the objects of the invention. Alliedwith this is fine grain structure which causes the articles to be ofdesirable impact strength. Experience has shown that'articles made inaccordance with the invention are highly resistant to scufllng andscoring, and provide better life than the materials heretofore used forthese purposes.

According to the provisions of the patent statutes I have explained theprinciple and method of practicing my invention, and have described whatI now consider to represent its best embodiment. However, I desire tohave it understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically described.

I claim:

1. A hot worked and heat treated article of manufacture, the articlebeing formed from steel containing about 1.0 to 2.0 per cent of carbon,about 0.3 to 2.0 per cent of molybdenum, about 0.4 to 0.8 per cent ofsilicon, about 2.0 to 6.0 per cent of tungsten, and the remaindereffectively iron, the article being characterized by high surfacehardness, by containing a substantial amount of its carbon content inthe graphitic state in highly dispersed form uniformly distributedthroughout the structure, by having spheroidized carbides uniformlydistributed throughout the structure, and by high resistance toscuffing, scoring and wear.

2. A hot worked and heat treated article of manufacture, the articlebeing formed from steel containing about 1.4 to 1.6 per cent of carbon,about 0.4 to 0.6 per cent of molybdenum, about 0.6 to 0.65 per cent ofsilicon, about 2.6 to 3.0 per cent of tungsten, and the remaindereffectively iron, the article being characterized by high surfacehardness, by containing a substantial amount of its carbon content inthe graphitic state in highly dispersed form uniformly distributedthroughout the structure, by having spheroidized carbides uniformlydistributed throughout the structure, and by high resistance toscuffing, scoring and wear.

3. That method of making graphitic steel articles which comprises hotworking an ingot of steel containing about 1.0 to 2.0 per cent ofcarbon, about 0.3 to 2.0 per cent of molybdenum, about 0.4 to 0.8 percent of silicon, about 2.0 to 6.0 per cent of tungsten, and theremainder effectively iron, annealing the hot worked article by heatingit above the critical, followed by cooling, to partially decomposecarbides and produce a substantial amount of graphitic carbon finelydispersed throughout the structure, and then reheating the graphitizedarticle to a temperature at least within the critical range tospheroidize residual cementite and thereafter cooling slowly to atemperature below the critical, and then heating the article to atemperature above the critical and quenching it to harden the articlewhile retaining substantially all of the graphitic carbon in that formand thereby rendering the article highly resistant to scufling, scoringand wear.

4. That method of making graphitic steel articles which comprises hotworking an ingot of steel containing about 1.4 to 1.6 per cent ofcarbon, about 0.4 to 0.6 per cent of molybdenum, about 0.6 to 0.65 percent of silicon, about 2.6 to 3.0 per cent of tungsten, and theremainder effectively iron, annealing the hot worked article by heatingit above the critical, followed by cooling, to partially decomposecarbides and produce a substantial amount of graphitic carbon finelydispersed throughout the structure, and then reheating the graphitizedarticle to a temperature at least within the critical range tospheroidize residual cementite and thereafter cooling slowly to atemperature below the critical, and then about 0.4 to 0.8 per cent ofsilicon, about 2.0 to

6.0 per cent of tungsten, and the remainder effectively iron, annealingthe hot worked article by heating it above the critical, followed bycooling, to partially decompose carbides and produce a substantialamount of graphitic carbon finely dispersed throughout the structure,and then reheating the graphitized article to a temperature at leastwithin the critical range to spheroidize residual cementite andthereafter cooling slowly to a temperature below the critical, and thenheating the article to a temperature above the critical and quenching itto harden the article while retaining substantially all of thegraphitic'carbon in that form and thereby rendering the article highlyresistant to scufiing, scoring and wear.

6. In a method of making graphitic steel articles, the steps whichcomprise casting an ingot of steel containing about 1.0 to 2.0 per centof carbon, about 0.3 to 2.0 per cent of molybdenum, about 0.4 to 0.8 percent of silicon, about 2.0 to

6.0 per cent of tungsten, and the remainder effectively iron, strippingsaid ingot and cooling it to atmospheric temperature, then reheatingsaid ingot above about 1600 F. and hot working it, annealing the hotworked article by heating it above the critical, followed by cooling, topartially decompose carbides and produce a substantial amount ofgraphitic carbon finely dispersed throughout the structure, and thenreheating the graphitized article to a temperature at least within thecritical range to spheroidize residual cementite and thereafter coolingslowly to a temperature below the critical, and then heating the articleto a temperature above the critical and quenching it to harden thearticle while retaining substantially all of the graphitic carbon inthat form and thereby rendering the article highly resistant toscufling, scoring and wear.

7. That method of making graphitic steel articles which comprises hotworking an ingot of steel containing about 1 to 2 per cent of carbon,about 0.3 to 2.0 per cent of molybdenum, about 0.4 to 0.8 per cent ofsilicon, about 2 to 6 per cent of tungsten, and the remainderefiectively iron, heating the hot worked article to a temperature of atleast about 1700" F. to decompose carbides and produce about 0.3 to 0.5per cent of graphitic carbon distributed throughout the structure, andcooling to a temperature below the critical range, reheating the articleto at least about 1400 F. to spheroidize residual cementite and thencooling slowly to about 900 F., then heating the spheroidized article toabout 1450 to 1550 F. and thereafter quenching it, and thereby renderingthe article of high surface hardness and of high resistance to wear,scufling and scoring.

FREDERICK R. BONTE.

